Quantitative testing for vitamin B12

ABSTRACT

Errors in the radioisotope dilution assay (RIDA) of the vitamin B 12  level of human blood have been traced to the presence of vitamin B 12  analogues. These previously unrecognized analogues bind with proteins normally included with binding proteins used in the prior art RIDA techniques. The errors caused by the B 12  analogues are avoided by using a composition for binding vitamin B 12  which is substantially free of substances which bind vitamin B 12  analogues. Moreover, by using both types of binders in separate assays, the amount of analogue present may be arrived at by taking the difference of the assays. Compositions that are effectively substantially pure intrinsic factor (IF) are used as binders.

The invention described herein was made, in part, in the course of work under a grant or award from the Department of Health, Education, and Welfare.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and materials for assaying mammalian blood and tissue. More specifically it relates to methods and materials for determining the amounts of vitamin B₁₂ and vitamin B₁₂ analogues in human plasma.

2. Prior Art

For many years it has been recognized that the assay of the vitamin B₁₂ level in humans is a valuable technique for diagnosing and subsequently treating certain diseases, such as for example, pernicious anaemia, post gastrectomy states nutritional deficiencies, intestinal disorders, and others. Initially, vitamin B₁₂ was assayed microbiologically using either Euglena gracilis or Lactobacillus leichmannii. More recently, radioisotope dilution (RID) assays for B₁₂ have been utilized. Such radioisotope dilution assay techniques are well documented in the literature, see for example Lau, et.al. (1965) "Measurement of Serum B₁₂ Levels Using Radioisotope Dilution and Coated Charcoal," BLOOD, 26, 202, as modified by Raven et.al. (1968) "The Effect of Cyanide Serum and Other Factors on the Assay of Vitamin B₁₂ by Radio-Isotope Method Using ⁵⁷ Co-B₁₂, Intrinsic Factor and Coated Charcoal," GUYS HOSPITAL REPORTS, 117, 89; and (1969) "Improved Method for Measuring Vitamin B₁₂ in Serum Using Intrinsic Factor, ⁵⁷ Co-B₁₂ and Coated Charcoal," JOURNAL OF CLINICAL PATHOLOGY, 22, 205.

Such prior art radioisotope dilution assay of vitamin B₁₂ generally includes the steps of freeing the endogenous B₁₂ from its natural binding protein by boiling at a selected pH and then adding a measured amount of the radioisotope ⁵⁷ Co-B₁₂ and a limited amount of binding protein. All of the binding protein will be bound by some form of B₁₂ since the amount of radioisotope B₁₂ added is, by itself, sufficient to bind the small amount of protein. As both the natural B₁₂ and the radioactive B₁₂ compete to bind with the protein, the degree to which the radioactive count of the protein bound B₁₂ was inhibited was thought to be indicative of the amount of natural B₁₂ present in the sample undergoing testing.

More specifically, in the technique of Lau et.al. as modified by Raven et.al., serum B₁₂ is separated from binding protein in the plasma sample by boiling with 0.25 N HCl. Radioisotope B₁₂ is added to the reaction mixture and the B₁₂ mixture is reacted with protein, normally in the form of a commercially available binder. Then the free or unbound B₁₂ is separated from the protein bound B₁₂ by protein-coated charcoal and the radioactivity of the supernatant liquid containing the mixture of bound radioactive B₁₂ and bound non-radioactive B₁₂ counted for radioactivity. The serum B₁₂ concentration is then calculated from the count, often by comparison with a standard chart. Almost as soon as this technique began to be utilized it was recognized that the vitamin B₁₂ measurements it provided were usually inconsistent with the results obtained by other measuring techniques for B₁₂, such as the microbiological assays. Most often, the vitamin B₁₂ assay obtained by radioisotope dilute techniques have been found to be high. Many theories have been advanced to explain the cause of the high vitamin B₁₂ readings. However, it is believed that nowhere in the prior art is it recognized that there are substances in mammalian blood and tissue which react with certain non-specific protein binders in the radioisotope dilution assay techniques to provide an analysis of vitamin B₁₂ which is apparently higher than the amount of B₁₂ actually in the sample. Additionally, it is believed that nowhere in the prior art is it recognized that most common and commercial RID assay protein binders are not specific to vitamin B₁₂, but that they are also capable of binding with the heretofore unknown B₁₂ analogues and thus provide erroneous B₁₂ assays.

BRIEF DESCRIPTION OF THE INVENTION

As has already been indicated, in the standard radioisotope binding assay for vitamin B₁₂, a known amount of radioactive vitamin B₁₂ is mixed with a prepared to-be-tested sample. Then, a known, but extremely limited, amount of protein which is capable of binding with both the natural and radioactive vitamin B₁₂ is added to the mixture. Then, utilizing well known techniques, the radioactivity of the bound sample is compared, for example, with a standard curve to determine the amount of natural vitamin B₁₂ present in the tested sample. Such standard curves are initially established for use in RID assay, for example, by measuring the amount of bound radioactive B₁₂ in the presence of the same type and amount of protein binder, but with several different amounts of known non-radioactive B₁₂.

It has now been discovered, for what is believed to be the first time, that mammalian blood and tissue contain materials other than vitamin B₁₂ which couple with certain binding proteins which are commonly used in RID assays. For purposes of this specification and claims the non-vitamin B₁₂ materials which are capable of binding with such proteins will be herein referred to as "vitamin B₁₂ analogues," "B₁₂ analogues" or simply as "analogues." They are referred to as analogues, not due to their chemical structure, which is not known with certainty, nor in the commonly accepted chemical sense of the word "analogue." Rather they are referred to as analogues due to their reactivity with the binding proteins commonly used in RID assays. As will be shown in more detail, hereinafter, there are other similarities which have been discovered between vitamin B₁₂ and the newly discovered analogues which are present in mammalian blood and tissue.

After the presence of B₁₂ analogues was discovered it was then determined that protein binders commonly present in RID assays were: (1) Non-specific in binding to only vitamin B₁₂ ; and (2) reactive in binding with both vitamin B₁₂ and B₁₂ analogues; and (3) capable of reacting with both B₁₂ and B₁₂ analogues independent of pH. These are most commonly R proteins. Additionally, it has been determined that other protein binders, are: (1) Very specific in their reactivity substantially only with vitamin B₁₂ ; (2) substantially non-reactive with the B₁₂ analogues; and (3) non-reactive with either vitamin B₁₂ or B₁₂ analogues in highly acid environments. These are most commonly proteins in the form of pure human intrinsic factor (IF), hog IF, rabbit IF, other IFs and vitamin B₁₂ specific binders.

In the past the problem has been that RID binders include substantial amounts of protein which is not specific to vitamin B₁₂. Therefore, the radioisotope dilution assay utilizing that binder on samples which contain B₁₂ analogues will produce a measurement which indicates a greater amount of B₁₂ present in the plasma than exists in fact. As will be shown in more detail hereinafter, commercially available protein binders, which have heretofore been labeled as containing intrinsic factor, in fact include only about 10% to about 30% intrinsic factor protein, while the balance of the protein in the binder is of a non-specific type, such as R protein. Thus, the protein materials in the commercial protein binders are capable of indiscriminate reaction with the heretofore unrecognized vitamin B₁₂ analogue materials in mammalian blood and tissue. These extraneous reactions give RID analyses having the appearance of apparently higher vitamin B₁₂ content than the samples in fact contain. This is due to the fact that when the binder includes protein which is non-specific to vitamin B₁₂ and which is capable of reacting with both vitamin B₁₂ and B₁₂ analogues, then the use of this protein in the radiobinding assay measures both the vitamin B₁₂ and the vitamin B₁₂ analogues which are present in the sample. However, in accordance with the present invention, when the proteins which are utilized are substantially specific to vitamin B₁₂, such as substantially pure intrinsic factor, then in the RID assay one binds and measures substantially only the vitamin B₁₂ in the sample, without the measurement of extraneous B₁₂ analogues. This provides a more accurate vitamin B₁₂ RID assay.

Based on these discoveries it is proposed that in the practice of RID assay only protein which is specific in its reaction to vitamin B₁₂ be utilized. Alternatively, it is proposed that mixtures of vitamin B₁₂ specific and non-specific binding proteins be treated, for example, with an excess of material such as B₁₂ analogues, which will bind or inactivate only the non-specific binding proteins, prior to the material's use in RID assays, so that the non-specific protein will be substantially unavailable for reaction with any vitamin B₁₂ or analogues in a sample when the RID assay is conducted. In yet another modification of the present invention, crude binder, including non-specific binding proteins, is subjected to proteolytic enzyme treatment prior to utilization as a vitamin B₁₂ binder in RID assays. Such proteolytic enzyme treatment destroys the binding ability of the non-specific proteins without destroying the binding ability of the proteins which are specific to vitamin B₁₂.

Utilizing the techniques of the present invention, the B₁₂ analogues can be assayed by analyzing the amount of vitamin B₁₂ present uitlizing, for example, a vitamin B₁₂ specific binder, then assaying the sample utilizing a non-specific binder and determining the difference between the two assays as a measure of the amount of vitamin B₁₂ analogues present.

These and other techniques are readily determined, once, as taught for the first time by the present invention, the presence of B₁₂ analogues in mammalian blood and tissue is recognized.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following examples and tables certain chemical components were utilized. For ease of communication they have been given shortened names in the text. The concordance between the "component" names and their actual compositions is as follows:

    ______________________________________                                         Components Actual Composition                                                  ______________________________________                                         A. Buffer  1.0M Tris (hydroxymethyl) aminomethane-                                        HCl pH 10.0                                                         B. Albumin Bovine serum albumin, 2 mg per ml in H.sub.2 O                      C. Salt    0.15M NaCl                                                          D. Boiled buffer                                                                          (1 part) 0.5M sodium acetate-HCl pH 4.5                                        (1 part) 0.01M KPO.sub.4 pH 7.5, 0.15M NaCl                                    (2 parts) 50 μg per ml KCN in 0.15M NaCl                                    The complete solution is heated for 45 min.                                    at 100° C.                                                   E. Standard                                                                               Solution D containing 100 pg per ml vitamin                         (100 pg/ml B.sub.12)                                                                      B.sub.12. The solution is heated for 45 min. at                                100° C. after the vitamin B.sub.12 is added. The                        concentration of vitamin B.sub.12 in the stock                                 solution used to make component E is deter-                                    mined by its light absorbance at 278, 361                                      and 550 nm.                                                         F. Standard                                                                               Same as component E except that the vitamin                         (1000 pg/ml B.sub.12)                                                                     B.sub.12 concentration is 1000 pg/ml.                               G. (.sup.57 Co) B.sub.12                                                                  1000 pg per ml of (.sup.57 Co) B.sub.12, (150-300                              uCi/ug), in H.sub.2 O.                                              H. Binder  Present in 0.01M Tris-HCl pH 8.2, containing                                   0.15M NaCl and 50 ug per ml bovine serum                                       albumin. Binders are diluted in this solu-                                     tion to reach a concentration of 700 pg per                                    ml of vitamin B.sub.12 binding ability.                                        Individual binders are as follows:                                             (1) Human intrinsic factor (Human (IF) -                                       Human gastric juice containing more                                            than 95% intrinsic factor based on                                             assays employing inhibition of vitamin                                         B.sub.12 binding with anti-intrinsic factor                                    antibodies (>95% inhibition) and co-                                           binamide (<5% inhibition).                                                     (2) Human R protein (Human R) - Human                                          saliva containing more than 95% R protein                                      based on assays employing inhibition of vita-                                  min B.sub.12 binding with cobinamide (>95%                                     inhibition) and anti-intrinsic factor                                          antibodies (<5% inhibition)                                                    (3) Hog intrinsic factor (Hog IF) - This                                       protein was purified from "Hog in-                                             trinsic factor concentrate" by affinity                                        chromatography on vitamin B.sub.12 -Sepharose                                  employing gradient elution with guani-                                         dine-HCl followed by gel filtration.                                           The final preparation contained more                                           than 95% intrinsic factor based on                                             assays employing inhibition of vitamin                                         B.sub.12 binding with anti-intrinsic factor                                    antibodies (>95% inhibition) and co-                                           binamide (<5% inhibition).                                                     (4) Hog R protein (Hog R) (Also designated                                     in the scientific literature as Hog non-                                       intrinsic factor) - This protein was                                           purified from "Hog intrinsic factor                                            concentrate" as described above in 3).                                         The final preparation contained more                                           than 95% R protein based on assays em-                                         ploying inhibition of vitamin B.sub.12 bind-                                   ing with anti-intrinsic factor antibod-                                        ies (<5% inhibition) and cobinamide                                            (>95% inhibition).                                                             (5) Rabbit intrinsic factor (Rabbit IF) -                                      An extract of rabbit gastric mucosa                                            containing more than 95% intrinsic fac-                                        tor based on assays employing inhibition                                       of vitamin B.sub.12 binding with anti-intrin-                                  sic factor antibodies (>95% inhibition)                                        and cobinamide (<5% inhibition).                                               (6) Hog intrinsic factor concentrate (Hog                                      IFC) - A crude extract of hog pyloric                                          mucosa. It contained 25% Hog IF and                                            75% Hog R based on assays employing                                            inhibition of vitamin B.sub.12 binding with                                    anti-intrinsic factor antibodies (25%                                          inhibition) and cobinamide (75% inhibi-                                        tion).                                                                         (7) Hog IFC + Cobinamide - Hog IFC contain-                                    ing the vitamin B.sub.12 analogue cobinamide                                   ([CN, OH]Cbi) in a molar amount equal                                          to 100 times the total vitamin B.sub.12                                        binding ability, i.e. a 100 fold excess                                        of cobinamide.                                                      The same as(8) Hog IFC + CN-Cbl [bde-OH]                                                  item (7) above except that the analogue                                        added is CN-Cbl [bde-OH] and is present                                        in a 1000 fold molar excess.                                                   (9) Hog IFC + [3,5,6-Me.sub.3 BZA] (CN, OH)Cba-                                The same as item (7) above except that                                         the analogue added is [3,5,6-Me.sub.3 BZA]                                     (CN, OH)Cba.                                                                   (10) Digested Hog IFC - Hog IFC incubated                                      with bovine pancreatic trypsin (2 mg                                           per ml) and bovine pancreatic chymo-                                           trypsin (2 mg per ml) for 60 min. at                                           37°  C.                                                      I. Charcoal                                                                               A solution containing 25 mg per ml neutral                                     charcoal (Norit) and 5 mg per ml bovine                                        serum albumin in H.sub.2 O.                                         J. unknown Samples containing unbound vitamin B.sub.12 are                     sample     diluted in solution D (see above). Samples                                     containing bound vitamin B.sub.12, such as serum,                              are prepared as follows:                                                       (1 part) sample                                                                (1 part) 0.5M sodium acetate-HCl pH 4.5                                        (2 parts) 50 ug per ml KCN in 0.15M NaCl                                       The complete mixture is heated at 100° C.                               for 45 min. followed by centrifugation                                         at 5000 × g at 4° C. for 20 min. The super-                       natant is removed and used for assay.                               ______________________________________                                    

Each of the RID assays referred to herein utilized the components referred to above. The method and order of utilizing the components is that set forth in Table I. That is, components A, B, C, etc. or the buffer, albumin, and salt, respectively, etc. were added in the order, from left to right, shown in Table I.

                                      Table I                                      __________________________________________________________________________     Flow Sheet for Radiobinding Assay for Vitamin B.sub.12                                           D   E      F       J                                               A   B    C  Boiled                                                                             Standard                                                                              Standard                                                                               Unknown                                                                              G     H   I                               Buffer                                                                             Albumin                                                                             Salt                                                                              Buffer                                                                             100pg/ml B.sub. 12                                                                    1000pg/ml B.sub.12                                                                     Sample                                                                               [.sup.57 Co] B.sub.12                                                                Binder                                                                             Charcoal                  Tube #                                                                               (μl)                                                                            (μl)                                                                             (μl)                                                                           (μl)                                                                            (μl)                                                                               (μl) (μl)                                                                              (μl)                                                                              (μl)                                                                            (μl)                   __________________________________________________________________________     1,2   225 50   875                                                                               800 --     --      --    50    --  --                        3,4   225 50   375                                                                               800 --     --      --    50    --  500                       5,6   225 50   325                                                                               800 --     --      --    50    50  500                        7    225 50   325                                                                               720  80    --      --    50    50  500                        8    225 50   325                                                                               640 160    --      --    50    50  500                        9    225 50   325                                                                               560 240    --      --    50    50  500                       10    225 50   325                                                                               300 500    --      --    50    50  500                       11    225 50   325                                                                               720 --      80     --    50    50  500                       12    225 50   325                                                                               640 --     160     --    50    50  500                       13    225 50   325                                                                               450 --     350     --    50    50  500                       14    225 50   325                                                                               --  --     800     --    50    50  500                       Unknown                                                                              225 50   325                                                                               --  --     --      800   50    50  500                       Sample                                                                         __________________________________________________________________________

After the addition of ⁵⁷ Co-B₁₂ the components are mixed thoroughly to mix both the naturally occurring B₁₂ and the radioisotope B₁₂ to make them compete and equally available to react with the binder. After the addition of the binder, H, the components were again mixed thoroughly, and then incubated for 45 minutes at about 37° C. Charcoal was then added to the incubated mixture and the components again mixed thoroughly and incubated for another 5 minutes at room temperature. This was followed by centrifuging at 2000×g at 4° C. for 30 minutes. Then 1000 ul of the resulting supernatant liquid is pipetetted from the sample and a determination of the amount of ⁵⁷ Co-B₁₂ present is made. The amount of ⁵⁷ Co-B₁₂ is indicative of the amount of natural B₁₂ in the tested sample, with lesser amounts of ⁵⁷ Co-B₁₂ being indicative of greater amounts of natural vitamin B₁₂ in the sample.

Calculations of vitamin B₁₂, utilizing the data obtained in the foregoing manner, is made as follows:

Calculation of data from radiobinding assay for plasma vitamin B₁₂ assay as outlined in Table I

(1) The values in tubes 3 and 4, the "blank" tubes without binder are averaged and subtracted from all other tubes starting with tube 5.

(2) The background radiation is subtracted from tubes 1 and 2 and these values are averaged.

(3) Tubes 5 and 6 are averaged. This value should be at least 15% below the average values for tubes 1 and 2 to insure that all of the binder is saturated in the presence of (⁵⁷ Co) B₁₂ alone.

(4) Values for each tube beginning with tube 7 are divided by the average of tubes 5 and 6 to give values for "% trace binding."

(5) Percent trace binding for tubes 7-14 are used to obtain a standard curve. We plot % trace binding on the ordinate of logit-log paper versus pg vitamin B₁₂ on the log scale.

(6) The amount of vitamin B₁₂ in unknown samples is determined by interpolation from the standard curve or data of % trace binding versus pg vitamin B₁₂.

(7) The standard curves for all of the various binders used are virtually indistinguishable and vary little from day to day. Nevertheless, a complete standard curve is always obtained for every binder with each set of assays. Representative data obtained with the assays are present in Table II.

                  Table II                                                         ______________________________________                                         Standard Curves obtained with the radiobinding                                  assay for vitamin B.sub.12 using various binders                                         % trace binding                                                                 observed with various binders.sup.(b)                              Vitamin B.sub.12 added.sup.(a)                                                              Human   Human   Hog   Hog   Hog                                   (pg)         IF      R       IF    R     IFC                                   ______________________________________                                         0            (100)   (100)   (100) (100) (100)                                 8            90      93      94    89    89                                    16           85      84      83    83    80                                    24           79      78      74    76    76                                    50           64      60      58    56    59                                    80           45      44      44    43    43                                    160          30      28      28    27    28                                    350          15      14      15    14    14                                    800           6       6       7     6     7                                    ______________________________________                                          .sup.(a) The vitamin B.sub.12 was boiled for 45 min. at 100° C. in      the same solution used to extract endogenous vitamin B.sub.12 from human       plasma.                                                                        .sup.(b) Assays were performed on different days.                        

Evidence as to the Origin and Existence of Vitamin B₁₂ Analogues in Mammalian Blood and Tissue

Once the problem of the prior art is recognized, that is, that there are vitamin B₁₂ analogues present in mammalian blood and tissue, it becomes a relatively simple matter to prove the existence and chemistry of such analogues. It is also appropriate to prove that the various steps of the RID assay do not cause the B₁₂ analogues to be formed, for example, from vitamin B₁₂.

In one instance this has been most convincingly shown by obtaining pure crystalline vitamin B₁₂, subjecting various known concentrations of it to the same conditions used to extract endogenous vitamin B₁₂ from blood and tissue samples (boiling for 45 minutes in the same extraction solution) and then analyzing them by RID assay using several binding proteins, for example, in the form of human IF, hog IF, human R, hog R and hog IFC on different portions of the same extracted vitamin B₁₂ samples.

Referring to Table II, it will be seen that when various known amounts of pure vitamin B₁₂, ranging from about 8 pg to about 800 pg were tested with various protein binders, that in each instance, the percent of radioactive trace binding, or more accurately, the inhibition of ⁵⁷ Co-B₁₂ binding, observed was substantially the same for each binder. It is thus seen, that regardless of which protein binder is utilized, the percent binding, i.e. inhibition of the (⁵⁷ Co)-B₁₂ is substantially the same. This is indicative of the fact that during preparation for RID assay the pure vitamin B₁₂ was not converted to analogues of the type which have now been observed in mammalian blood and tissue. It is also indicative of the fact that in the absence of interferring masking components in the samples, such as B₁₂ analogues, any of the binding proteins can be utilized to provide substantially equally accurate RID assays of vitamin B₁₂. Furthermore, the data in Table II should be suitable as a standard in the determination of vitamin B₁₂ by the same RID assay.

By comparison, when endogenous vitamin B₁₂ was extracted from serum from 74 normal blood donors (37 women, 37 men, ages 17-61) and tested utilizing the same binding proteins with the exception of hog IFC which was not used, the results were quite different. In every case in which serum from normal donors was tested greater inhibition of ⁵⁷ Co-B₁₂, and therefore greater apparent vitamin B₁₂, was observed with assays employing, as the binder, human R or hog R than was observed with assays employing human IF or hog IF. The data on the 74 normal donors is included in Table III.

                                      Table III                                    __________________________________________________________________________     Distribution of a serum vitamin B.sub.12 values as measured with various       binders for 74 normal subjects and 21 patients                                 with clinical evidence of vitamin B.sub.12 deficiency                          All assays were performed at pH 9.0                                                                           Human R                                                                              Hog R                                                                    minus minus                                     Serum  Human R                                                                              Hog R Human IF                                                                             Hog IF                                                                               Human IF                                                                             Hog IF                                    Vitamin B.sub.12                                                                      Nor-                                                                              Pa-                                                                               Nor-                                                                              Pa-                                                                               Nor-                                                                              Pa-                                                                               Nor-                                                                              Pa-                                                                               Nor-                                                                              Pa-                                                                               Nor-                                                                              Pa-                                    (pg/ml)                                                                               mals                                                                              tients                                                                            mals                                                                              tients                                                                            mals                                                                              tients                                                                            mals                                                                              tients                                                                            mals                                                                              tients                                                                            mals                                                                              tients                                 __________________________________________________________________________      0-24                 8     11                                                 25-49                 6     5                                                  50-74                 5     3     1  2  1                                      75-99     1     1     2     2  2     3                                         100-124   2     1              2  3  9  3                                      125-149   2     5  4     1     7  3  5  3                                      150-174   3     1  5     3     6  5  9  5                                      175-199   4     3  2     3     6  3  8  3                                      200-224                                                                               1  1     2  5     4     8  2  11 2                                      225-249   3  1  1  6     4     2  3  6  3                                      250-274         3  10    9     10    4                                         275-299   1  2  1  4     3     5     1                                         300-324                                                                               1  2  4  2  12    4     6     3                                         325-349                                                                               3  1  2  1  5     6     4  1  5  1                                      350-374                                                                               5  1  1           10    4     1                                         375-399                                                                               3     2     3     4     4     2                                         400-424                                                                               5     3     4     5     5     1                                         425-449                                                                               3     4     3     2     2                                               450-474                                                                               4     6     2     3           1                                         475-499                                                                               5     5           3           1                                         500-524                                                                               7     4     1     3                                                     525-549                                                                               3     4     1                                                           550-574                                                                               5     3     2     1           1                                         575-599                                                                               1     2     1           1                                               600-624                                                                               2     4     1     3           1                                         625-649                                                                               5     4                                                                 650-674                                                                               3     3     2                                                           675-699                                                                               2     3                                                                 700-724                                                                               3     6           1                                                     725-749                                                                               2                                                                       750-774                                                                               3     1           1                                                     775-799      1     1                                                           800-824      2                                                                 825-849                                                                        850-874                                                                               1     1                                                                 875-899                                                                               1     2           1                                                     900-924                                                                               1     1                                                                 925-949                                                                               2     1                                                                 950-974                                                                               1                                                                       975-999                                                                               1                                                                       1000-1024                                                                      1025-1049                                                                      1050-1074                                                                             1                                                                       1075-1099                                                                      1100-1124                                                                      1125-1149    1                                                                 1150-1174                                                                      1175-1199                                                                      1200-1224                                                                      1225-1249                                                                             1                                                                       range of                                                                              220-                                                                              85-                                                                               245-                                                                              84-                                                                               130-                                                                              0- 135-                                                                              0- 70-                                                                               56-                                                                               50-                                                                               72-                                    serum B.sub.12                                                                        1230                                                                              355                                                                               1135                                                                              342                                                                               785                                                                               78 880                                                                               86 575                                                                               337                                                                               605                                                                               332                                    mean of                                                                        serum B.sub.12.sup.(a)                                                                576                                                                               205                                                                               573                                                                               204                                                                               322                                                                               36 361                                                                               33 254                                                                               169                                                                               212                                                                               171                                    mean ± 2                                                                           282-  276-                                                                              136-  157-  70-   58-                                          Std. Dev.                                                                             1040  1065  656   717   661   531                                       __________________________________________________________________________      .sup.(a) Based on values obtained with log vitamin B.sub.12. Log values        were used since these gave a normal curve while the curve using                untransformed values were skewed to the right.                           

Other data concerning patients with diagnosed vitamin B₁₂ deficiencies are present, and comparisons between the normal donors and patients have also been made on Table III, and will be discussed in more detail hereinafter. Referring to Table III it is seen that the mean endogenous vitamin B₁₂ RID assay levels, in terms of pg of vitamin B₁₂ per ml of serum, are 548 and 542 for human R and hog R, respectively, but only 298 and 336 for human IF and hog IF, respectively, This demonstrates that something is present in extracts of normal human serum which inhibits the vitamin ⁵⁷ Co-B₁₂ binding ability of human R and hog R to a greater extent than it inhibits the binding ability of human IF and hog IF. Under current RID assay techniques the greater inhibition which is found using human R and hog R is analyzed to indicate a higher vitamin B₁₂ content. It is those substances, which have now been found to be present in human blood serum and which preferentially inhibit ⁵⁷ Co-B₁₂ binding of human R and hog R, which have been herein denominated as "vitamin B₁₂ analogues."

Chemical Nature and Properties of Vitamin B₁₂ Analogues

The vitamin B₁₂ analogues, which are herein for the first time identified as being present in mammalian blood and tissue, have been isolated by paper chromatography and compared with pure vitamin B₁₂. Vitamin B₁₂ and the so-called "vitamin B₁₂ analogues" were found to have the following properties in common: (1) Both were adsorbed to charcoal and remained adsorbed when the charcoal was washed with 5% phenol; (2) Both were eluted from charcoal when the charcoal was washed with 67% acetone; (3) Both were extracted from aqueous solution into phenol and remained in the phenol phase even when the phenol was washed repeatedly with water; (4) Both passed into the aqueous phase when the phenol layer was dissolved in an excess of diethyl ether; (5) Both eluted with similar apparent molecular weights (approximately 1356) during gel filtration on columns of Bio-Rad P-4 polyacrylamide; (6) Both were adsorbed to a column of Sepharose-2B agarose that contained covalently bound hog R protein and both remained bound when the column was washed with 0.1 M glycine-NaOH pH, 10.0, 1.0 M NaCl, and both were eluted from the Sepharose with either 85% phenol or 60% pyridine. Because of these similarities the newly discovered material is seen to be similar to vitamin B₁₂ and is thus referred to as vitamin B₁₂ analogue.

The chemical nature and structure of the newly discovered vitamin B₁₂ analogues which are now found to be present in mammalian blood and tissue is not known. An effort was made to compare them with chemically ture forms of vitamin B₁₂, sometimes referred to in the literature as analogues of vitamin B₁₂, namely CN-B₁₂, OH-B₁₂, adenosyl-B₁₂ and CH₃ -B₁₂, already known to be present in serum and tissues. This was done by adding 500 pg of each of these four known forms of vitamin B₁₂ to four different portions of the same human serum, in the dark. Prior to the additions the serum contained 250 pg and 450 pg of vitamin B₁₂ as assayed by RID using human IF and human R, respectively, thus exhibiting a difference of 200 pg. After addition of the materials to the serum, each was allowed to incubate in the dark for 15 minutes to allow binding of the added known forms of vitamin B₁₂ to the binding proteins normally present in the serum. Then the serum, with the added forms of vitamin B₁₂ was extracted utilizing standard conditions and the apparent amount of vitamin B₁₂ assayed by RID utilizing both human R protein and human IF protein. Both the human R and human IF assays showed an increase in the apparent amount of vitamin B₁₂ of about 500 pg. However, the original difference observed between the values obtained with the human R protein and the human IF protein, i.e. 200 pg, did not change. If any of the added known forms of vitamin B₁₂ in the human serum had been converted to the newly discovered analogues, then the assays would have shown an increase in the difference. This provides evidence that the newly discovered vitamin B₁₂ analogues were not formed from any of the known endogenous forms of native vitamin B₁₂ during the extraction procedure.

Isolation of Vitamin B₁₂ Analogue

The materials which are herein designated as "vitamin B₁₂ analogues" and which have been found to preferentially inhibit R proteins in the vitamin B₁₂ assays were substantially separated from endogenous vitamin B₁₂ by the following purification shceme. A trace amount of 150 pg ⁵⁷ Co-B₁₂, was added to 1800 ml of freshly collected normal human plasma. The added ⁵⁷ Co-B₁₂ was sufficiently small that it did not interfere with subsequent RID assays. After incubating at room temperature for 30 minutes the vitamin B₁₂ was extracted and assayed under standard conditions. When human IF binder was utilized in the RID assay the extract was found to contain 1050 ng of vitamin B₁₂, but when human R protein was utilized as the binder it appeared to contain 2030 ng of vitamin B₁₂, almost twice as much vitamin B₁₂. The extract was then passed through a column of Sepharose containing covalently bound hog R protein. The column retained greater than 99% of the ⁵⁷ Co-B₁₂ as well as the endogenous vitamin B₁₂ as assayed by RID with human IF or human R protein. After the column was washed with a variety of buffers and water the material was eluted with 60% pyridine, taken to dryness under vacuum, dissolved in water, and adsorbed onto charcoal. The charcoal was washed with 5% phenol followed by water and the mixture of vitamin B₁₂, ⁵⁷ Co-B₁₂ and analogue B₁₂ was eluted from the charcoal with 67% acetone. The material was again taken to dryness under vacuum, dissolved in water, and then separated utilizing 19 inch long Whatman 3 MM paper for paper chromatography and a solvent system consisting of 800 ml sec-butanol, 8 ml glacial acetic acid, 6 mmol HCN and 400 ml water. The chromatography was performed in the descending manner for 30 hours at room temperature in an environment that inhibited evaporation of the solvent. The paper chromatogram was allowed to dry in a fume oven and divided into 38 one-half inch fractions and numbered, with fraction 1 starting at the point of applicaion and number 38 being at the lowest point on the chromatogram. Each one-half inch fraction was then incubated with 5 ml of water at 4° C. for twelve hours to elute the vitamin B₁₂, ⁵⁷ Co-B₁₂ and B₁₂ analogues. The water was then removed and taken to dryness under vacuum. Each dried fraction was then dissolved in 2.5 ml of water and assayed for ⁵⁷ Co-B₁₂ and for vitamin B.sub. 12 using a variety of binding proteins. The final recovery of ⁵⁷ Co-B₁₂ was 64%. The apparent recoveries of vitamin B₁₂ were 75% when using human IF in the assay and 66% when using human R in the assay. The results of the assays employing the 38 fractions obtained by paper chromatography are presented in Tables IVA, IVB, and IVC. Similar data concerning paper chromatography of ⁵⁷ Co-B₁₂ and pure vitamin B₁₂, for reference as a control, are presented in Table IVD. The data in these several chromatography tables summarized for convenience in Table IVE reveals that the behavior of ⁵⁷ Co-B₁₂ that was extracted from human plasma did not change its chromatographic behavior, and thus was not altered during the standard extraction procedure or any of the purification steps. In a similar manner it is postulated that true vitamin B₁₂ is not altered in any of the purification or process steps of the assay.

                                      Table IVA                                    __________________________________________________________________________     [.sup.57 Co] B.sub.12 and plasma B.sub.12 assayed after elution from           paper chromatography.                                                          All assays were performed at pH 9.0 except where indicated.                    B.sub.12 assayed with various binders                                                             pH 1.8       pH 1.8                                         [.sup.57 Co] B.sub.12                                                                       Human R                                                                              Human R                                                                              Hog R Hog R Hog IFC                                   Fraction #                                                                           (cmp)                                                                              (%)                                                                               (ng)                                                                              (%)                                                                               (ng)                                                                              (%)                                                                               (ng)                                                                              (%)                                                                               (ng)                                                                              (%)                                                                               (ng)                                                                              (%)                                    __________________________________________________________________________     1            2     1     1     1     1                                         3                                                                              4            1     1     1     2     1                                         5            2           2     3     2                                         6            3     3     2     4     2                                         7            4     4     4     5     3                                         8            4     3     4     5     3                                         9            7  1  6     6  1  8  1  5 1                                       10    300 1  24 2  29 2  26 2  29 2  16                                                                               2                                       11    500 2  42 3  54 4  36 3  49 3  26                                                                               3                                       12    300 1  25 2  34 2  26 2  38 2  19                                                                               2                                       13    200 1  31 2  44 3  30 3  55 4  22                                                                               2                                       14    4600                                                                               20 209                                                                               16 235                                                                               16 176                                                                               16 240                                                                               16 180                                                                              17                                      15    14000                                                                              61 562                                                                               42 615                                                                               43 461                                                                               41 576                                                                               37 474                                                                              46                                      16    2100                                                                               9  103                                                                               8  122                                                                               9  84 7  122                                                                               8  91                                                                               9                                       17    300 1  85 6  118                                                                               8  85 8  128                                                                               8  53                                                                               5                                       18    300 1  68 5  68 5  68 6  100                                                                               6  45                                                                               4                                       19    200 1  23 2  17 1  17 2  27 2  15                                                                               1                                       20           45 3  39 3  39 3  60 4  24                                                                               2                                       21           42 3  25 2  25 2  52 3  21                                                                               2                                       22           8  1  5     7  1  8  1  5 1                                       23           8  1  4     5     8  1  4                                         24           7  1  4     5     7     3                                         25           9  1  1     6  1  3     4                                         26           16 1  1     10 1  4     5 1                                       27           5     1     3     2     2                                         28           2           1     1     1                                         29           4           2     1     2                                         30           1           1           1                                         31           1                                                                 32                                                                             33                                                                             34                                                                             35                                                                             36                                                                             37                                                                             38                                                                             __________________________________________________________________________

                  Table IVB                                                        ______________________________________                                         Assay of [.sup.57 Co] B.sub.12 and plasma B.sub.12                             after elution from paper chromatography.                                       All assays were performed at pH 9.0.                                                        B.sub.12 assayed with various binders                             [.sup.57 Co] B.sub.12                                                                         Human IF  Hog IF    Rabbit IF                                   Fraction #                                                                             (cpm)   (%)    (ng) (%)  (ng) (%)  (ng) (%)                            ______________________________________                                         1                      1                   1                                   3                                                                              4                                                                              5                                1         1                                   6                                1                                             7                                1         1                                   8                                1         1                                   9                      1         1         2                                   10      300     1      4    1    5    1    8    1                              11      500     2      7    1    10   1    16   2                              12      300     1      5    1    7    1    10   1                              13      200     1      14   2    10   1    13   1                              14      4600    20     169  21   169  22   170  19                             15      1400    61     488  61   450  57   480  55                             16      2100    9      73   9    72   9    87   10                             17      300     1      17   2    25   3    42   5                              18      300     1      11   1    15   2    25   3                              19      200     1      3         5    1    5    1                              20                     5    1    7    1    8    1                              21                     2         4    1    4                                   22                                                                             23                                                                             24                                                                             25                                                                             26                                                                             27                                                                             28                                                                             29                                                                             30                                                                             31                                                                             32                                                                             33                                                                             34                                                                             35                                                                             36                                                                             37                                                                             38                                                                             ______________________________________                                    

                                      Table IVC                                    __________________________________________________________________________     Assay of [.sup.57 Co] B.sub.12 and plasma B.sub.12 after elution from          paper chromatograpy.                                                           All assays were performed at pH 9.0.                                                        B.sub.12 assayed with various binders                                                Hog            Hog IFC +                                                       IFC + Hog IFC +                                                                               [3,5,6-Me.sub.3 BZA]                                                                   Hog IFC                              [.sup.57 Co] B.sub.12                                                                       Hog IFC                                                                              Cobinamide                                                                           CN-Cbl[bde-OH]                                                                          (CN, OH)Cba                                                                            Digested                             Fraction #                                                                           (cpm)                                                                              (%)                                                                               (ng)                                                                              (%)                                                                               (ng)                                                                              (%)                                                                               (ng)  (%)                                                                               (ng) (%)                                                                               (ng)                                                                              (%)                               __________________________________________________________________________     1            1                            1                                    3                                                                              4            1                            1                                    5            2           1        1       1                                    6            2           1        1       1                                    7            3           1        1       2                                    8            3           1        1       2                                    9            5  1  1     1        2       3                                    10    300 1  16 2  4     5     1  7   1   9  1                                 11    500 2  26 3  10 1  10    1  13  2   15 2                                 12    300 1  19 2  8  1  8     1  9   1   11 1                                 13    200 1  22 2  9  1  10    1  12  2   14 2                                 14    4600                                                                               20 180                                                                               17 174                                                                               21 170   22 148 20  153                                                                               19                                15    14000                                                                              61 474                                                                               46 474                                                                               58 435   55 396 53  402                                                                               50                                16    2100                                                                               9  91 9  83 10 81    10 80  11  84 10                                17    300 1  53 5  25 3  27    3  34  4   36 4                                 18    300 1  45 4  16 2  19    2  21  3   27 3                                 19    200 1  15 1  4     4     1  6   1   9  1                                 20           24 2  7  1  7     1  11  1   13 2                                 21           21 2  3     4     1  7   1   9  1                                 22           5  1                 1       2                                    23           4                    1       1                                    24           3                            1                                    25           4                        2                                        26           5  1                         2                                    27           2                                                                 28           1                            1                                    29           2                                                                 30           1                                                                 31                                                                             32                                                                             33                                                                             34                                                                             35                                                                             36                                                                             37                                                                             38                                                                             __________________________________________________________________________

                  Table IVD                                                        ______________________________________                                         Assay of [.sup.57 Co] B.sub.12 and native B.sub.12                             after elution from paper chromatography.                                       All assays were performed at pH 9.0.                                                        B.sub.12 assayed with various binders                             [.sup.57 Co] B.sub.12                                                                         Human R   Human IF  Hog IFC                                     Fraction #                                                                             (cpm)   (%)    (ng) (%)  (ng) (%)  (ng) (%)                            ______________________________________                                         2                                                                              3                                                                              4                                                                              5                                                                              6                                                                              7                                                                              8                                                                              9                                                                              10      200     1      1                   1                                   11      600     2      1         1         2    1                              12      400     1      1         1         2    1                              13      400     1      4    1    3     1   6    2                              14      6800    24     86   25   86   26   90   27                             15      15800   56     210  60   198  60   189  56                             16      3100    11     40   12   38   12   40   12                             17      200     1      2    1    1         3    1                              18      200     1      2    1    1         2    1                              19      100            1                   2    1                              20                                                                             21                                                                             22                                                                             23                                                                             24                                                                             25                                                                             26                                                                             27                                                                             28                                                                             29                                                                             30                                                                             31                                                                             32                                                                             33                                                                             34                                                                             35                                                                             36                                                                             37                                                                             38                                                                             ______________________________________                                    

                                      Table IV E                                   __________________________________________________________________________     Summary of the data in Tables IVA - IVD involving assays of [.sup.57 Co]       B.sub.12 and B.sub.12 after elution                                            from paper chromatography. Assays for B.sub.12 were performed at pH 9.0        except where indicated                                                                                     Chromatogram Fractions                                          Item  Assay    1-13  14-16 17-38 1-38                             Sample Chromatographed                                                                      Assayed                                                                              Binder   (ng)                                                                              (%)                                                                               (ng)                                                                              (%)                                                                               (ng)                                                                              (%)                                                                               (ng)                                                                              (%)                           __________________________________________________________________________     [.sup.57 Co] B.sub.12 + B.sub.12                                                            [.sup.57 Co] B.sub.12                                                                 --      -- 5  -- 92 -- 2  -- 100                           "            B.sub.12                                                                             Human R   7 2  336                                                                               97  5 1  348                                                                               100                           "            B.sub.12                                                                             Human IF  5 1  322                                                                               98  2 1  329                                                                               100                           "            B.sub.12                                                                             Hog IFC  11 3  319                                                                               95  7 2  337                                                                               100                           [.sup.57 Co] [.sup.57 Co] B.sub. 12ub.12                                                           .sub.[  -- 6  -- 91 -- 3  -- 100                           "            B.sub.12                                                                             Human R  145                                                                               11 874                                                                               65  324                                                                              24 1343                                                                              100                           "            B.sub.12                                                                             Human R  179                                                                               12 972                                                                               68  283                                                                              20 1434                                                                              100                                              (pH 1.8)                                                    "            B.sub.12                                                                             Hog R    138                                                                               12 721                                                                               64  274                                                                              24 1133                                                                              100                           "            B.sub.12                                                                             Hog R    199                                                                               13 938                                                                               61  402                                                                              26 1539                                                                              100                                              (pH 1.8)                                                    "            B.sub.12                                                                             Hog IFC  100                                                                               10 745                                                                               72  185                                                                              18 1030                                                                              100                           "            B.sub.12                                                                             Human IF 31 4  730                                                                               91 38 5  799                                                                               100                           "            B.sub.12                                                                             Hog IF   37 5  691                                                                               88 56 7  784                                                                               100                           "            B.sub.12                                                                             Rabbit IF                                                                               53 6  737                                                                               84 84 10 874                                                                               100                           "            B.sub.12                                                                             Hog IFC +                                                                               32 4  731                                                                               89 54 7  818                                                                               100                                              Cobinamide                                                  "            B.sub.12                                                                             Hog IFC +                                                                               47 6  624                                                                               83 81 11 752                                                                               100                                              CN-Cbl[bde-OH]                                              "            B.sub.12                                                                             Hog IFC +                                                                               38 5  686                                                                               87 61 8  785                                                                               100                                              [3,5,6-Me.sub.3 BZA]                                                           (CN, OH)Cba                                                 "            B.sub.12                                                                             Hog IFC -                                                                               60 7  639                                                                               80  102                                                                              13 801                                                                               100                                              Digested                                                    __________________________________________________________________________

Referring to the control chromatogram of Table IVD, it is seen that the several RID assays of pure vitamin B₁₂ performed variously with human R protein, human IF protein and hog IFC gave substantially a single symmetrical peak of activity. In each instance greater than 95% of the vitamin B₁₂ was found to be present in fraction 14 through 16. Similar results, as shown in Table IVB, were obtained from the paper chromatogram of the plasma extract when the binding protein was human IF, hog IF and rabbit IF. These data are an indication that these three IF binding proteins are substantially specific in their binding ability to vitamin B₁₂, and substantially non-reactive with vitamin B₁₂ analogues present in plasma.

Efforts were made to modify hog IFC, which is a commonly used binder in RID assays and which has been found to contain as much as 90% hog R protein and as little as 10% hog IF, by removing or inactivating the hog R. In several instances the hog IFC was incubated with an excess amount of three chemically synthesized vitamin B₁₂ analogues before it was utilized in the RID assay. Referring to Table IVC, it is seen that after this modification the chromatogram results obtained utilizing the modified hog IFC closely resemble the results obtained with substantially pure hog IF. It is therefore seen, that in the practice of the present invention, mixtures of protein including both vitamin B₁₂ specific binding protein and binding protein which is not specific to vitamin B₁₂ can be modified by the addition of an excess amount of vitamin B₁₂ analogue, by which process the analogue binds with the non-specific protein to render it substantially bound or inactive so that it is not available to react with vitamin B₁₂ or (⁵⁷ Co) B₁₂ present in samples undergoing RID tests. The amount of vitamin B₁₂ analogue to be added to a mixture of specific and non-specific proteins in order to bind or inactivate the non-specific proteins may vary over a wide range, depending on both the proteins which are present and the vitamin B₁₂ analogues which are utilized as the binding or inactivating material. Generally speaking, for the examples shown in Table IVC, cobinamide may be added in an amount equal to that required for complete binding of the non-specific protein, up to amount as much as ten million times greater than the amount needed to bind the protein, with the preferred range being about ten to about ten thousand times in excess of that required for complete binding. CN-Cb1 (bde-OH), known as CoB-cyano-cobamic a,c,g-triamide may be utilized in an amount at least about ten times to about ten million times in excess of the amount required to bind with the non-specific protein, with an amount of about one hundred to about one hundred thousand times excess being preferred. The (3,5,6-Me₃ BZA) (CN,OH)-cba, known as Coα(3,5,6-trimethylbenzimidazole) cobamide should also be utilized in amounts from about one to ten million times in excess of the amount of non-specific protein, with an amount in the range of about ten to about ten thousand times excess being preferred. Suitable amounts of other vitamin B₁₂ analogues may be utilized in a similar manner to bind or inactivate non-specific proteins present in mixtures with specific proteins in order to obtain a preparation of binding protein which will substantially bind only ⁵⁷ Co-B₁₂, or the vitamin B₁₂ naturally present in the samples being tested, and thus give a more accurate quantitative RID assay of vitamin B₁₂ in samples undergoing tests.

Again, referring to Table IVC, data on samples of hog IFC digested with trypsin and chymotrypsin are shown. These and other proteolytic enzymes are specific in their ability to substantially digest R proteins while leaving intrinsic factor proteins unaffected and available as substantially the only protein for binding ⁵⁷ Co-B₁₂ and vitamin B₁₂ in RID assays. Other enzymes, including, for example, elastase may be utilized for the same purpose. The amount of the enzymes utilized is in the range of about 0.01 to about 100 milligrams per mililiter of protein treated, with a preferred amount being about 0.05 to about 40 milligrams per mililiter of protein. Utilizing this proteolytic enzyme digestion process a protein binder is provided which substantially binds only ⁵⁷ Co-B₁₂ and vitamin B₁₂ and is not affected by vitamin B₁₂ analogues in the samples being tested and which therefore gives a more accurate RID assay than is obtained when utilizing the original mixture of hog IFC proteins which included non-specific proteins which would have been capable of reacting with the newly discovered vitamin B₁₂ analogues in samples to give inaccurate assays as to the amount of vitamin B₁₂ in test samples.

Now, referring to Table IVA, when samples from the plasma extract chromatogram were assayed for vitamin B₁₂ with human R, hog R and hog IFC, different results were obtained than when those samples were assayed with human IF, hog IF, rabbit IF or hog IFC treated with vitamin B₁₂ analogues or hog IFC digested with proteolytic enzymes. In each case where human R, hog R or untreated hog IFC were utilized as the binding protein the tests gave the appearance that more vitamin B₁₂ was present in the chromatogram samples, especially in fractions 1 through 13 and 17 through 38. This observation, when taken with the above data, provides strong evidence that normal human plasma contains a number of vitamin B₁₂ analogues that compete with ⁵⁷ Co-B₁₂, in significant amounts, for binding to R protein. It also indicates that such activity on the part of the B₁₂ analogues is substantially absent when the binding protein utilized in the RID assay is substantially specific to vitamin B₁₂.

It should also be noted, see Table IVA, that the chromatogram data suggests that the lack of specificity of human R and hog R is unchanged when RID assays are performed at acid pH. This indicates that erroneous results will be obtained for the true vitamin B₁₂ content of samples which contain vitamin B₁₂ analogues when RID assays are performed at acid pH.

Using the same techniques and criteria described above it has been discovered that vitamin B₁₂ analogues are not only present in serum obtained from human blood, but are also present in mammalian tissues in even higher concentrations than they are in blood. Vitamin B₁₂ analogues extracted from mammalian tissues have been purified using the same schemes as described above. When analyzed utilizing paper chromatography, they exhibited similar mobilities to those of the vitamin B₁₂ analogues observed in the samples extracted from blood serum. Since larger amounts of the vitamin B₁₂ analogues are present in tissue, they can be observed visually as red or orange spots during paper chromatography. The absorption spectra of vitamin B₁₂ analogues purified from tissue extracts have been determined and demonstrate that they are similar to, but distinct from, the absorption spectrum of true vitamin B₁₂. These observations provide additional evidence that the materials in blood serum which preferentially react with R proteins and not intrinsic factor proteins are in fact varieties of vitamin B₁₂ analogues.

The newly discovered vitamin B₁₂ analogues also differ from vitamin B₁₂ in terms of their biological activity. Thus, as shown in Table V, the serum vitamin B₁₂ values obtained with Euglena gracilis for eleven patients diagnosed to have vitamin B₁₂ deficiency were substantially similar to the results obtained by RID assay using human IF or hog IF as the binding protein.

                                      Table V                                      __________________________________________________________________________     Vitamin B.sub.12 levels in 21 patients with clinical evidence of vitamin       B.sub.12 deficiency                                                                    Vitamin B.sub.12 assayed with various binders                                                       Vitamin B.sub.12 assayed                                  Human R                                                                             Hog R                                                                               Human IF                                                                             Hog IF                                                                              with Euglena Gracilis                             Patient (pg/ml)                                                                             (pg/ml)                                                                             (pg/ml)                                                                              (pg/ml)                                                                             (pg/ml)                                           __________________________________________________________________________      1      155  138  16     0   43                                                 2      310  295  51    86   52                                                 3      310  255  24    35   46                                                 4      135  132   0    22   40                                                 5      215  250  52    60   25                                                 6      347  342   0     0   56                                                 7      102  120   0     0   35                                                 8      240  242  58    65   79                                                 9      160  125  38    23    0                                                10       85   84   0    12   23                                                11      235  255   5    11    0                                                12      188  190  41    38                                                     13      178  192  48    38                                                     14      298  305  53    57                                                     15      355  310  78    82                                                     16      178  210  48    22                                                     17      128  140  31     8                                                     18      106  190  50    40                                                     19      163  155   0     0                                                     20      178  132  25     0                                                     21      230  215  75    41                                                     Mean (1-11)                                                                            209  203  22    29   36                                                Mean (1-21)                                                                            205  204  33    30                                                     Normal Range                                                                           220-1230                                                                            245-1230                                                                            136-656                                                                              157-717                                                                             (>130)                                            (mean ± 2                                                                   std. dev.)                                                                     Number within                                                                          9 (43%)                                                                             10 (48%)                                                                             0     0    0                                                normal range                                                                   __________________________________________________________________________

It is to be further noted, that all of the values obtained by either microbiologic assay or by assay using human IF or hog IF were substantially lower than the values obtained when the RID assay was carried out utilizing human R or hog R as the binding protein. This indicates that the vitamin B₁₂ analogues which have now been identified in mammalian blood and tissue do not possess vitamin B₁₂ activity of the type which is required to promote the growth of Euglena gracilis.

Data was obtained on ten additional patients diagnosed to be vitamin B₁₂ deficient and the total of 21 patients with vitamin B₁₂ deficiency are shown in Table V. In each of the 21 patients the vitamin B₁₂ values found when the RID assay was carried out utilizing human IF or hog IF were below the range of vitamin B₁₂ values found in a control group of 74 normal subjects. However, when the RID assay was carried out utilizing human R or hog R only about half of the 21 vitamin B₁₂ deficient patients were found to assay below the range of normal subjects for vitamin B₁₂. This indicates that where the newly found vitamin B₁₂ analogues are present in the samples being tested, and the binding protein is not specific to vitamin B₁₂, the resulting assays may suggest that a truly vitamin B₁₂ deficient patient is not within the deficient range. This may lead to delay of treatment of that patient for vitamin B₁₂ deficiency. It also indicates that the vitamin B₁₂ analogues that have now been discovered lack the therapeutic or beneficial activity of vitamin B₁₂ in the sense of being unable to prevent the hematologic and/or neurologic abnormalities associated with vitamin B₁₂ deficiency.

There are many commercial RID assay type kits available for the assay of vitamin B₁₂ in clinical laboratories. Table VI sets forth an analysis of several such kits, and a comparison of the types of protein found in those kits with hog IF, hog R and hog IFC. By reference to Table VI, it is seen that the commercial kits appear to have only about 13% to about 35% intrinsic factor and from about 60% to about 85% R protein. It is therefore suspected, that the use of these kits will give substantially erroneous assays of the amount of vitamin B₁₂ present in a sample when the sample also includes vitamin B₁₂ analogues, such as those newly discovered to exist in mammalian blood and tissues. It has also been determined that the effectiveness of intrinsic factor to bind vitamin B₁₂ is somewhat pH dependent, with intrinsic factor losing about 10% of its binding ability at a pH of about 4.1 and losing about 99% of its binding ability at a pH of about 1.9. Thus, to the extent that the commercial kits use a pH of about 4.1 during binding they would have about 10%l less intrinsic factor than shown in Table VI. Those kits having a pH during binding of about 1.7 to about 1.9 would obtain substantially no binding from intrinsic factor.

                                      Table VI                                     __________________________________________________________________________     Analysis of vitamin B.sub.12 binders and assay pH used in commercial kits      sold for the                                                                   assay of vitamin B.sub.12 in clinical laboratories                                             Vitamin B.sub.12 Binding Protein                                               intrinsic factor.sup.(a)                                                                R protein.sup.(b)                                     Source of Binder                                                                               (%)      (%)   Assay pH of kit.sup.(d)                         __________________________________________________________________________     Hog IF          97        0    --                                              Hog R            0       98    --                                              Hog IFC         25       75    --                                              Diagnostic Products Corp. Kit                                                                  35       60    1.7                                             New England Nuclear Kit                                                                        20       82    4.1                                             Bio-Rad Laboratories Kit                                                                       30       71    1.9                                             Medvak Diagnostic Products Kit                                                                 13       85    1.8                                             Schwarz/Mann Kit                                                                               34       67    9.1                                             Pharmacia Diagnostics Kit                                                                       1 (33).sup.c                                                                           67    4.1                                             __________________________________________________________________________      .sup.(a) Inhibition of [.sup.57 Co] B.sub.12 binding observed with antiIF      antibodies at pH 7.5.                                                          .sup.(b) Inhibition of [.sup.57 Co] B.sub.12 binding observed with a 100       fold molar excess of cobinamide at pH 7.5.                                     .sup.(c) In this kit the binder is covalently attached to an insoluble         matrix and because of steric factors it may not be accessible to               antiintrinsic factor antibodies. Thus the value for % intrinsic factor ma      be as high as 33%.                                                             .sup.(d) Refers to the pH measured after the addition of all of the            components of each individual kit. R protein retains its full B.sub.12         binding ability over the range of pH 1.7-9.1 but intrinsic factor loses        10% and 99% of its B.sub.12 binding ability % intrinsic factor than is         indicated in the table while those that use pH 1.7-1.9 are employing           essentially no intrinsic factor.                                         

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other modifications or changes in form and details may be made therein without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A radioisotope dilution assay for measuring the vitamin B₁₂ level in a sample comprising:contacting said sample with a known amount of a radioisotope of vitamin B₁₂ and a composition containing a binding protein substantially specific to vitamin B₁₂, said composition being substantially free of substances which bind vitamin B₁₂ analogues.
 2. The method of claim 1 wherein the source of vitamin B₁₂ is selected from the group consisting of mammalian tissue and mammalian blood.
 3. The method of claim 1 wherein the source of vitamin B₁₂ is human blood.
 4. The method of claim 1 wherein the binding protein is intrinsic factor.
 5. The method of claim 4 wherein the binding protein consists essentially of protein selected from the group consisting of human IF, hog IF and rabbit IF.
 6. The method of measuring vitamin B₁₂ analogues present in a sample which also includes vitamin B₁₂, including the steps of:assaying a portion of the sample with a binder which is a binder for both vitamin B₁₂ and vitamin B₁₂ analogues; assaying a portion of the sample with a binder which is substantially specific to vitamin B₁₂ and substantially free of substances which bind vitamin B₁₂ analogues; and then determining the difference between the two assays as being substantially indicative of the amount of vitamin B₁₂ analogue in the sample. 